The present invention relates, in general, to communication systems and, particularly, to an optical wavelength multiplexer/demultiplexer usable in connection with wavelength division multiplex (WDM) optical communication.
A number of optical communication devices and systems are configured for use with optical wavelength division multiplexing (WDM). In WDM, several information signals can be transmitted over a single optical fiber. Each information signal is used to modulate a different wavelength carrier signal or channel. Many operations in such systems require multiplexing or demultiplexing the signals. Multiplexing generally involves combining multiple channels into a single WDM signal. Demultiplexing generally involves extracting or separating individual channels, e.g. for subsequent processing such as routing to desired pathways and/or outputting to output ports. Each channel occupies or is contained within a predefined frequency range. In many (but not all) systems the frequency ranges defining the boundaries for the channels are equal in bandwidth, evenly spaced and contiguous. Since the total bandwidth for a WDM signal cannot be significantly smaller than the sum of the bandwidths of the component channels, for a given bandwidth WDM signal, the number of channels that can be defined (and thus the number of separate information signals that can be carried) increases as the bandwidth of the channels decreases. Thus it is not surprising that systems have tended toward increasingly-smaller channel spacing to increase the number of optical channels. Many systems would, accordingly, benefit from optical devices which can successfully accommodate channel bandwidths of, for example, 100 GHz, 50 GHz or even less.
Unfortunately, as the number of channels is increased, the channel spacing is decreased so that functions such as demultiplexing/multiplexing have become increasingly difficult. In addition to the difficulty of constructing devices with sufficient accuracy to, even theoretically, demultiplex such narrow-bandwidth channels, and without wishing to be bound by any theory, it is believed narrow-bandwidth demultiplexing is particularly susceptible to factors such as wavelength drift and/or channel cross-talk.
Accordingly, it would be useful to provide a demultiplexer/multiplexer which can provide useful results with respect to narrow-bandwidth channels such as 100 GHZ, 50 GHz channels or smaller, preferably while being relatively tolerant of wavelength drift and/or providing relatively low channel cross-talk.
It is, in general, believed possible to construct devices which permit channels to be controllably provided to desired output ports (i.e. routers) and/or permit a new information signal to replace an existing information signal in a channel of a WDM (i.e. an add/drop device). It is believed some such devices involve active (i.e. controlled) channel separation in which binary or other control signals function to achieve the desired routing or add/drop configuration. It is believed that many such systems are configured such that a change in the control signals (i.e. a change in routing or add/drop function) affects (e.g. interrupts or xe2x80x9ctouchesxe2x80x9d) all or substantially all of the channels in the WDM signal. While this may be acceptable for some applications, it is believed other applications (such as asynchronous, continuous, real time and/or time critical signals) would be more readily served by devices which permitted re-routing or add/drop changes of only desired channels while leaving other channels in a WDM signal substantially unchanged or untouched.
Accordingly, it would be useful to provide a demultiplexer/multiplexer, and associated devices such as add/drop devices, routers, and the like, particularly narrow-bandwidth channel systems, in which the multiplex /demultiplex function was substantially passive and/or in which channels not being changed by a change in routing or add/drop function would be substantially unaffected or untouched.
The present invention includes a recognition of problems of previous approaches, including as described above. The present invention involves a wavelength filter which operates by providing different polarizations to contiguous channels. Thus, if a plurality of channels are, in order, assigned sequential numbers, odd-numbered channels will be provided with a first polarization while even numbered channels are provided with a second polarization. Such change in polarization permits the odd-numbered and even-numbered channels to be placed on different optical paths. In this way, the present invention provides for a single device or stage that separates a plurality of non-contiguous channels, all at once, from remaining channels. The separation of formerly xe2x80x9cinterdigitatedxe2x80x9d channels means that the channels in each path are separated by a channel spacing twice the separation between channels of the original WDM signal (for the typically evenly-spaced channels). The system is scalable in the sense that each of the two resulting double-spaced channel signals can be further separated into, e.g., two signals containing alternately-numbered channels which are quadruply spaced. For example, an original WDM signal has channel separation of 100 Gigahertz (GHz), a first iteration of a wavelength filter as described herein will result in two signals (one carrying channels 1, 3, 5, 7 . . . , the other carrying channels 2, 4, 6, 8 . . . ) with the channels of each of the two signals being separated by 200 GHz. A second iteration of wavelength filtering (using a different filter, as described below) will result in two signals arising from each of the first and second filtered signals, for a total of four signals (the first having channels 1, 5, 9 . . . , the second having channels 3, 7, 11 . . . , the third having channels 2, 6, 10 . . . , the fourth having channels 4, 8, 12 . . . ) with the channels in each of these four signals being separated from adjacent channels by 400 GHz. The process may be repeatedly iterated sufficiently to achieve desire separation of channels. For example it is at least theoretically possible to iterate until there is one channel per signal (i.e. the WDM signal has been fully demultiplexed) using the described wavelength filters, or more conventional demultiplexing devices (conventional wavelength filtering) and methods can be applied to resultant signals (502, 503 or 504, 505, 506, 507, or the like) in which the increased channel separation permits successful application of conventional demultiplexing technology, preferably with acceptable wavelength drift tolerance and acceptably low cross talk. Although a xe2x80x9cchannelxe2x80x9d may refer to a single frequency range used to carry a single information signal, it is possible to decompose or demultiplex a WDM signal only partially such that one or more of the outputs of the demultiplexer may include more than one channel and may itself be a WDM signal (with a smaller bandwidth than the original WDM signal).
Because the demultiplexing is provided in the absence of control signals and is substantially passive, the demultiplexed signals are provided as substantially continuous, uninterrupted signals. Accordingly, in one embodiment the demultiplexer can be used in conjunction with other devices to achieve, e.g., add/drop functionality in a manner such that changing the add/drop function (such as providing an add signal to a different channel) can be achieved without affecting, interrupting or xe2x80x9ctouchingxe2x80x9d any of the channels other than those involved in the change.